As the insulation level of a transformer increases, the insulation arrangement of its high voltage terminals gains importance. The matter is not only the insulation between the terminals and earth, but also between any pair of terminals in the same winding. This mainly applies to the lightning impulse withstand voltage, although the power frequency withstand voltage also plays a role. The problem of the insulation can be viewed in two ways:
On one hand, the higher the voltage, the more difficult it is also to provide sufficient insulation against earth and between terminals in the same winding. Also, the smaller the dimensions, the more difficult the insulation is between terminals in the same winding. The inclusion of barriers around a terminal or the covering of its surface with solid insulation increases the electric field (and so the voltage) it can support without having any discharge. The effect of the barriers can be explained with their property of stopping free charges which can initiate a discharge, while the effect of the solid insulation can be explained with its lower electron emissivity compared with a metal. Apart from that, in both cases the creepage distance is increased, thus contributing to a greater withstand voltage.
Regarding HV terminals for cast-coil dry-type transformers, the following types are usually applied. The terminals for the lines connection often consist of bared bolts, which can be placed at the top and bottom edges of the phase. Usually, the terminals have no special insulation, or they may have grooves in order to increase the creepage distance against earth potential or other live points in the same winding. Further, smooth bushings may be applied, which increase the creepage distance. Known are also bushings that are equipped with additional sheds, e.g. for high levels of pollution or even for outdoor installation. In the case of tap-changer terminals, consisting of groups of bared bolts placed in the middle of the winding, there is typically no special insulation applied around them. However, also in this case, protrusions, grooves, or even bushings may be applied.
When a series connection is applied to connect windings, e.g. when there is more than one winding in the same magnetic core leg, the same arrangements as for the tap-changer terminals can be used for interconnecting the windings to each other.
Particularly at high voltages or difficult environmental conditions, the known techniques may suffer from various isolation issues. Further, if such issues are addressed by employing bushings or the like, enhanced production cost will result and enhanced risk of damage can result, e.g. during transportation of the transformer.
U.S. Pat. No. 3,569,884 discloses transformer coils wound from sheet conductor and cast together with their high-voltage lead conductors in a resin housing. The high-voltage lead conductors are braced against the low voltage windings. This allows to reduce the possibility that stresses are applied to the housing through the rigid high-voltage lead conductors. GB 1 602 970 and AU 521 297 alike disclose transformer coils wound from sheet conductor and cast together with their rigid high-voltage leads in a resin housing. US 2009/0284338 discloses a transformer with a multi-stage coil made of flat rectangular wires. In view of the above, there is a need for the present invention.